Is there something which his regeneration doesn't/can't heal him from? Because I rem u saying about his wounds from his torture that are still there

EDIT:

And it was released as a LN almost a year ago, it seems to be selling nicely

Spoiler for ...:

Hes hard to kill, but he can't regenerate instantly fatal injuries. As for Tatsuya's scars, I have only speculation on my part. They probably blocked his magic somehow when they punished him, or the damage was done before his magic had gotten to the level it did.